Germanium hollow cathode assembly for lamps



Dec. 2, 1969 J. H. VOLLMER I 3,482,138

GERMANIUM HOLLOW CATHODE ASSEMBLY FOR LAMPS Filed May 16, 1967 INVENTOR. Jozz H. Wilmer United States Patent 3,482,138 GERMANIUM HOLLOW CATHODE ASSEMBLY FOR LAMPS John H. Vollmer, Norwalk, Conn., assignor to The Perkin- Elmer Corporation, Norwalk, Conn., a corporation of New York Filed May 16, 1967, Ser. No. 638,850 Int. Cl. H01j 1/05, 1/14, 1/48 U.S. Cl. 313-311 4 Claims ABSTRACT OF THE DISCLOSURE This invention relates to an improved hollow cathode used in a lamp, of the type especially useful as a light source in spectroscopic instruments of the atomic absorption type. More particularly this invention concerns an improved hollow cathode for a lamp, which is useful primarily for atomic absorption measurements of, for example, germanium.

In atomic absorption spectroscopy the sample (containing one or more metals) is analyzed by determining the absorption (at a certain specific wavelength of radiation) caused by the atoms for which the analytical test is being made. Usually the metallic sample is vaporized in the flame of a burner, so that the sample is atomized, and then irradiated with a light source of great intensity at least at one characteristic absorption band of the testedfor metal. Only transmitted radiation in the region of this characteristic wavelength is then allowed to reach a detector, which therefore yields a measurement of how much absorption has occurred. The detected intensity (as compared to the original source intensity, for example) yields a quantitative measurement of the absorption and therefore the concentration of the particular metal for which the analysis is being conducted. A description of atomic absorption spectroscopy and spectrometers is contained in United States Patent No. 2,847,899, issued Aug. 19, 1958, to A. Walsh.

In order to irradiate the sample at high intensity in the narrow absorption band, the light source itself preferably includes a relatively high concentration of the metal for which the test is being made. A typical such light source is the hollow cathode lamp, in which a cupshaped element (including at least a substantial percentage of the metal for which the test is intended) acts as the negative electrode of the lamp. Both this hollow cathode and the positive electrode are hermetically sealed within a glass envelope in a low pressure atmosphere of an inert (noble) gas. A typical such hollow cathode lamp is shown in applicants pending United States patent application Ser. No. 591,748, filed Nov. 3, 1966.

As stated in said pending application Ser. No. 591,748, the actively radiating part of the hollow cathode should have certain desirable physical characteristics to provide high intensity spectral radiation, to have a long useful life, and to simplify manufacture. The use of an alloy or other mixture including the desired tested-for metal, either for the whole cathode or as an interior coating of the cathode cup, can improve the physical characteristics (e.g., melting point, vapor pressure at operating 3,482,138 Patented Dec. 2, 1969 temperature, manufacturing workability, and the like) of the active part of the cathode.

In addition to having such desirable mechanical properties and physical and chemical stability at all temperatures to which it is likely to be exposed in use, the actively emitting cathode material must also have certain desirable electrical characteristics. In particular this cathode materi- 211 should have an electrical resistance of a convenient valve (i.e., relatively low) under the actual operating conditions of the lamp, and the resistance-versus-temperature curve (over the range from room temperature through operating temperature) should be smooth. Otherwise, the lamp may undergo relatively sudden or large changes in intensity even when driven by a well regulated power supply. In fact for materials exhibiting a relatively sharp change in their effective electrical resistance with varying temperature, the current through (and therefore the radiation intensity of) the lamp may oscillate, even though connected to a constant voltage source. For purposes of simplicity, the term unstable electrical resistance will be used to refer to a material, the electrical resistance or impedance-versus-temperature curve of which has any one of the following: (a) substantial change in slope; (b) a point of inflection; or (c) a discontinuity at any temperature within the range of interest. An example of a metal with such unstable electrical resistance characteristics is germanium.

Another important property of the material that is to form the active emission cloud inside the hollow of the cathode is the so-called work function of this material. The work function measures the energy necessary to cause particles (ideally atoms) to be liberated from the solid mass of the cathode material to form free atoms. It is important that the work function have a value compatible with the operating temperature and voltage of the lamp, if fairly high intensity of the spectral line radiation is to be obtained relative to the electrical input to (and life of) the lamp.

The present invention provides an improved hollow cathode lamp, in which the active hollow cathode material is of the type that would in its pure form exhibit unstable electrical resistance characteristics (for example, germanium) by intimately mixing this material with another material (for example, copper), thereby obtaining a resulting alloy mixture which has substantially stable and favorable electrical resistance characteristics.

An object of the invention is accordingly the provision of an improved hollow cathode assembly for use in a lamp emitting radiation in the characteristic spectrum of a metal having unstable electrical resistance characteristics (e.g., germanium).

A related object is the provision of such a hollow cathode, which allows the lamp to be run at higher current, and therefore provides greater radiation energy, than has been previously practical with existing lamps.

Another object is the provision of an improved spectral radiation source lamp for the atomic absorption analysis of germanium.

Other objects and advantages of the invention will become obvious to one skilled in the art upon reading the following detailed description of an exemplary embodiment of the invention in conjunction with the accompanying drawing, in which:

The sole figure is an enlarged vertical cross section through the improved hollow cathode cup or holder, incorporating in its interior an alloy mixture according to the invention.

The cathode assembly 20 includes the cathode cup or holder 22 having a narrow apertured portion 24 which may be press-fit or crirnped upon the cathode pin of the hollow cathode lamp (for example, pin 18 of FIG. 1 of said application Ser. No. 562,145). The wider or cup portion of the cathode 26 will include as an interior coating at 28 the substance (for example, a germaniumcopper alloy, as hereinafter described) which determines the actual radiation of the lamp.

A major point of novelty of the present lamp is the constituency of the actively emitting substance 28. In the following it will be assumed that the lamp is intended to be used as a spectral source of germanium atomic radiation. As will appear however, the principle of the invention may be utilized for sources of different spectral radiation. For a germanium lamp, substance 28 is preferably a mixture containing substantially 40% germanium and 60% copper (by weight). Such a mixture is substantially at the eutectic point of these two metals, and presumably consists of a mixture of the true alloy Cu Ge and some excess germanium.

Approximately one gram of the desired mixture may be made and cast as a single step within a hollow cathode cup holder (of the type shown at 22-26), which cathode cup holder is preferably of substantially pure nickel. This mixing and casting may be conveniently accomplished by placing the above-mentioned proportions (e.g., 0.4 and 0.6 gram, respectively) of germanium and copper in the upright, empty hollow cathode (i.e., with its open end 23 uppermost) in an inert atmosphere (e.g., argon gas). The entire assembly is then raised to approximately 650 C., and cast by spinning. These operations are most conveniently accomplished within a covered centrifuge, having provision for heating. The temperature is raised to a point above the melting point (640) of the eutectic mixture, and then spinning the cathode cup assembly until the resulting liquid climbs up the then vertical cylindrical walls 25 of the cathode cup. The germanium-copper mixture will form a coating on the internal surface of the cup having a generally parabolic internal surface at 29-. The spinning is maintained after the heat source has been turned off, so that the cooling mixture 28 maintains this shape.

Because of the eutectic nature of the CUgGB alloy and excess germanium mixture, the composition of the substance at the operative surface 29 of the active material 28 remains fairly constant even after substantial use of the lamp. The germanium-copper alloy mixture is vastly superior to, for example, pure germanium for a number of reasons. Pure germanium is very brittle and has a low coefficient of thermal expansion; it therefore has a marked tendency to form cracks and distintegrate both during casting and when it is repeatedly heated and cooled (as of course necessarily occurs during any intermittent lamp use).

The preferred mixture has markedly improved electrical characteristics relative to pure germanium. Germanium of course exhibits semiconductive properties, having a relatively high electrical resistance (for a metal), which resistance appreciably changes with both varying temperature and effective applied voltage. Because of this unstable electrical characteristic, previous (pure) germanium lamps of this type have exhibited varying radiation intensity even when connected to a constant voltage source. In fact such previous lamps often pulsate or oscillate (at a period on the order of, say, three seconds) in intensity. Such oscillation causes both substantial shortening of useful lamp life and marked increase in the noise level of the radiation detector system (for example, in an atomic absorption spectrometer) with which it is used. Lamps utilizing the germanium-copper alloy mixture of the invention do not exhibit such oscillation even when run at relatively high intensity (for example, 30 ma.). The alloy mixture of the invention not only possesses a much smoother electrical resistance-versus-temperature curve, but the electrical resistance values are also substan tially lower than that of pure germanium in the temperature range of interest. Additionally the effective work function (for germanium) is lower with the inventive mixture than for pure germanium. The lowered electrical resistance and work function greatly facilitate the obtaining of stable high radiation intensities, made possible by the increased stability of the electrical characteristics.

Obviously the exact proportions of the germanium and copper may be varied somewhat from the preferred 40% and 60% figures. However for a simple two-constituent alloy mixture, the copper should be within the range of about 55 and 65% (the germanium therefore being between 35% and The invention has been described with reference to a specific mixture (within a moderate range) of germanium and copper, so as to yield a lamp specifically suited to spectral emission of germanium (which lamp is particularly adapted for use as a source in atomic absorption spectroscopy and similar uses). However the principle of the invention may be utilized for different materials having similar unstable electrical characteristics (for example, antimony, which also exhibits unstable electrical characteristics because of its semiconductive nature; silicon, and the like). Generally a metal of relatively low, stable electrical resistance will be added to such electri cally unstable elements so as to form a mixture (generally including at least in part an alloy) having the desired stable, moderately low electrical resistance. For example, antimony may be alloyed with copper by utilizing approximately 75% antimony and 25% copper.

As may be seen from the preceding detailed description, the invention accomplishes its major purpose of providing an improved hollow cathode assembly for a lamp emitting radiation in the characteristic spectrum of a metal having unstable electrical characteristics. A complete example of one specific embodiment providing spectral radiation for germanium has been disclosed. Since, as previously noted, the principle of the invention may be adapted to provide hollow cathode lamps for spectral radiation of other elements having unstable or otherwise unsuitable electrical characteristics, and since various specific details may obviously be varied by one skilled in the art, the invention is not limited to either such material or details. On the contrary the invention is defined solely by the scope of the appended claims.

What is claimed is:

1. In a hollow cathode assembly for a spectral radiation lamp, said hollow cathode assembly being of the type comprising a substantially cup-shaped holder having an interior coating thereon, which coating comprises a particular metal for which spectral radiation is desired, the improvement in which:

said metallic interior coating comprises a mixture containing a substantial quantity of germanium and a substantial quantity of copper intimately intermingled therewith so as to form a substantially homogeneous mixture;

said germanium comprising at least about 30% of said mixture by weight;

and said copper comprising at least about of said mixture by weight;

said resulting mixture being substantially free of the unstable electrical properties of germanium, caused by its semi-conductive characteristics,

whereby an improved stable bright cathode source of spectral radiation for germanium is obtained.

2. An improved hollow cathode assembly according to claim 1, in which:

said cup-shaped hollow cathode holder constitutes at least substantially pure nickel.

3. An improved hollow cathode assembly according to claim 1, in which:

said germanium comprises between 35% and 45% of said mixture by weight;

and said copper comprises substantially all the rest of said mixture, that is, between about and by weight.

6 An improved hollow cathode assembly according to 3,089,054 5/1963 Walsh et al. 313218 X claim 3, in wh ch: 3,183,393 5/1965 Paterson 313218 X said germanium comprises about 40% of said mixture y Weight, FOR I PATE said mixture therefore being substantially the eutectic E GN NTS of Cu Ge alloy and Ge, 0 977,545 12/ 1964 Great Britain.

whereby said mixture tends to maintain substantially the same percentage composition even after sub- JOHN HUCKERT, Pnmary EXamlnel' stantial quantities of said mixture have been lost from said interior coating through sputtering and the like 10 JAMES Asslstant Exammer d ar'bllamu.

urmg pp ecla e p Se US. Cl. X.R.

References Cited UNITED STATES PATENTS 3,020,436 2/1962 Ahsmann 313-218 X 15 

